Method for firing ceramic honeycomb bodies

ABSTRACT

The invention relates to a method of firing a green ceramic honeycomb structural body containing an organic or carbonaceous material which is characterized by removing at least a portion of the carbonaceous material released prior to reacting within the firing atmosphere during the initial firing of the green honeycomb structural body. The invention also relates to a tunnel kiln, which includes a vestibule region, a carbonaceous material release region, having a plurality of removal zones, located downstream of the vestibule region and a sintering region located downstream of the carbonaceous material release region. The tunnel kiln further includes an exhaust removal system which operatively communicates, via offtake openings located in each removal zone, with the release region for removing released carbonaceous material.

This application is a continuation-in-part of application Ser. No.09/203,614, filed Dec. 1, 1998, now U.S Pat. No. 6,089,860, which claimsthe benefit of U.S. Provisional Application Serial No. 60/068,487, filedDec. 22, 1997, entitled “METHOD FOR FIRING CERAMIC HONEYCOMB BODIES ANDA TUNNEL KILN USED THEREFOR”, by Dull et al.

The present invention relates to a method of firing cellular ceramicbodies, and more particularly, it relates to a method of firing thecellular ceramic bodies exhibiting problematic high-organic containingbatches and to a tunnel kiln adapted for such a firing process.

BACKGROUND OF THE INVENTION

Ceramic products of a honeycomb shape, or ceramic honeycomb structures,i.e., cellular ceramic bodies, have been made by preparing a ceramicgreen body through mixing of ceramic materials with water and variouscarbonaceous materials, including extrusion and forming aids to form aplasticized batch, forming the body into a honcycomb-shaped ceramicgreen body through extrusion of the plasticized batch, and finallyfiring the honeycomb-shaped ceramic green body in a firing furnace at apredetermined temperature.

Extrusion and forming aids used in the above firing of the honeycombstructure include, specifically, organic binders and plasticizers andlubricants, such as methylcelloluse, carboxymethlcellulose, polyvinylalcohol, alkali stearates and the like. Furthermore, other carbonaceousmaterials such as graphite have been included in the batch as apore-forming agent.

It is known that the carbonaceous material release or the decompositionof the carbonaceous material, is an oxidation or exothermic reactionwhich releases large amounts of heat. Initially the exothermic reactionoccurs at the skin or outer portion of the part, resulting in an initialthermal differential whereby the outer portion of the ceramic body ishotter than the core. Subsequently, the skin or outer portion exothermicreaction dies down, and the exothermic reaction region moves into theinterior of the ware. Because typical substrates are comprised ofceramic materials, for example cordierite, which are good insulators,and exhibit a cellular structure comprising numerous channels,difficulties are encountered in effectively removing, either byconduction or convection, the heat from the ceramic body. Additionally,due to the cellular structure there is considerable surface area topromote binder reaction with the O₂ in the firing atmosphere, thusexacerbating this interior exothermic effect. As such, during thecarbonaceous material release, the ceramic body exhibits either apositive or negative thermal differential; i.e., the core of the ceramicbody exhibiting either a higher or lower temperature than that of theceramic at/near the surface. This exothermic reaction, which occurs inthe 100 to 600° C. temperature range for carbonaceous materials such asan organic binder or the like, or in the 500-1000° C. temperature rangeif the body contains, for example, graphite, causes a significanttemperature differential between the inside and outside of the part.This temperature differential in the part creates stresses in theceramic body which may result in cracking of the part. This phenomenonis particularly true for large cellular ceramic parts or partscontaining large amounts of organic materials.

Techniques for controlling and inhibiting the thermal differential andresultant crack development are well known. One technique involvesreducing burner flame temperature by using excess air for burnercombustion, resulting in a reduced flame to product temperature gradientand corresponding slower ware heating rates. However, the high excessair yields an undesirably high percentage oxygen-containing atmospherethat reacts with the organics thereby accelerating release andincreasing the internal exothermic reaction. As such, minimization ofthe thermal differential which develops during organic release, must beaccomplished through very slow firing schedules or, alternatively,firing schedules which are carefully matched to the particular ware inthe kiln.

Use of atmosphere control in periodic-type kilns to affect carbonaceousmaterial release is generally known. See, for example, U.S. Pat. Nos.,4,404,166 (Wiech, Jr.), 4,474, 731 (Brownlow et al.), 4,661,315 (WiechJr. et al.) and 4,927,577 (Ohtaka et al.). Although these methods havebeen shown to be effective enough for use in periodictype kilns, theyare not generally considered to be effective in tunnel kilns due to theconsiderable influx of ambient air (20.9% oxygen) into the firingatmosphere.

The use of pulse firing technology as a substitute for proportionalfiring has also been disclosed as a method for controlling andinhibiting thermal gradients in periodic kilns. Pulse firing involvesthe use of either high fire or low fire burner output conditions only,and produces low heating rates without the use of considerable amountsof excess air (oxygen); see, for example Eur. Pat. Appl. No. 0 709 638which discloses a method of firing ceramic formed bodies using a furnacehaving burners which alternate from a high to a low output firing state.Although the use of this firing technology has been somewhat effectivein periodic kilns, resulting in a reduction in the incidences ofcracking, this pulse firing technique poses difficulties when used intunnel kilns. Due to the open nature of tunnel kilns it is necessary tocontrol the ambient air ingress into the organic release zones of thekiln by other means.

Therefore, an object of the invention is to solve the above-mentionedproblems of the prior art by providing an improved method and a tunnelkiln for firing ceramic honeycomb structural bodies which ensures stableproduction of high-quality crack-free product.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate theabove-mentioned problems, and to provide a process and a kiln for firingceramic honeycomb structural bodies, which permits the production ofceramic honeycomb structural bodies exhibiting less cracks, a moreuniform firing of the inner and outer portions of the green honeycombstructural bodies while employing short duration cycles.

The method of firing a green ceramic honeycomb structural bodycontaining an organic or carbonaceous material is characterized bycollecting and trapping, via the use of a collection trap, at least aportion of the carbonaceous material released from the body. The methodnext involves, subsequently, and prior to the material reacting withinthe firing atmosphere of the kiln, removing this collected/trappedreleased carbonaceous material.

The kiln according to the present invention is a tunnel kiln whichcomprises the following: a vestibule region; a carbonaceous materialrelease region having a plurality of removal zones and associatedcarbonaceous material collection trap regions which are locateddownstream of the vestibule region; and a sintering region locateddownstream of the carbonaceous material release region. The tunnel kilnfurther includes an exhaust removal system which operativelycommunicates with the release region for removing released carbonaceousmaterial. The exhaust removal system comprises at least one offtakeopening in communication with a corresponding carbonaceous materialcollection trap, preferably located in the rooftop of each removal zoneand operatively communicating with an exhaust fan, for removing andevacuating released and trapped carbonaceous material from the kilnrelease region.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the invention, reference is made to theattached drawings, wherein:

FIG. 1 is diagram representative of the differences in temperaturebetween the core and skin of a ceramic honeycomb structural body formedand fired by conventional firing methods;

FIG. 2 is a schematic view illustrating a tunnel kiln apparatus suitablycapable of being used to carry out the process for firing the ceramichoneycomb structural bodies according to the present invention;

FIG. 3 is a schematic view illustrating another embodiment of a tunnelkiln apparatus suitably capable of being used to carry out the processfor firing the ceramic honeycomb structural bodies according to thepresent invention;

FIGS. 4A-4C are partial sectional views, along line A—A of FIG. 3,illustrating a series of removal zones and the various embodiments ofthe collection traps useable in the inventive tunnel kiln apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The ceramic-firing process and apparatus according to the presentinvention are based on knowledge obtained by examining conventionalfiring kilns and their firing conditions. Conventional firing proceduresused to convert the plasticized batch or ceramic green body into aceramic product typically results in a temperature differential betweenthe outer surface or skin and the inside or core due to the exothermicrelease of the organic or carbonaceous material extrusion or formingaids included in the starting batch. This organic or carbonaceousrelease occurs between about 100-600° C. for organic binders andextrusion aids or between about 500-1000° C. for the graphite-likematerials. While the heat generated at the outer periphery or skin ismore easily dissipated, though still sufficient to cause stresses whichmay exceed the strength of the part, the heat generated in the core ofthe ceramic body is more troublesome as it is not dissipated due to thecellular structure and the insulative nature of the cordieriteceramicbody.

FIG. 1 illustrates a typical, undesired skin/core temperature profile,of a conventionally fired cordierite-ceramic honeycomb body; thistemperature differential is such that the fired body produced tends toexhibit thermally induced deformation as well as firing cracks. Ascellular bodies exhibit thinner cell walls and exhibit greater celldensities, and as more and different organic binders and graphite-likematerials are used to maintain the structural integrity of thesecellular bodies, this phenomenon is likely to increase.

This invention provides an effective method of producing fired honeycombceramic structural bodies, substantially free of any detrimental effectsas a result of the release of the carbonaceous material. Specifically,the method comprises, prior to sintering, firing the ceramic green bodyto a temperature and for a time sufficient to substantially achieve therelease of the carbonaceous material while at the same time alsocollecting/trapping and subsequently removing at least a portion of thereleased carbonaceous material prior to its downstream movement andsubstantial reaction in the firing atmosphere of the kiln.

The collection/trap feature prevents the gases from drifting downstreaminto higher temperature zones thereby avoiding situations where thegases would likely ignite and subsequently result in undesirabletemperature or oxygen level kiln/firing atmosphere conditions; i.e.,out-of-control temperature situations.

This invention may be applied to any ceramic material which may bedetrimentally affected by carbonaceous material release; typical ceramicmaterials include, for example, and without limitation, cordierite andalumina-containing ceramics. The invention is hereinafter described interms of a cordierite-containing ceramic honeycomb material, however,this should not be considered as limiting the invention to that ceramicmaterial.

Raw materials for ceramic batches useful in the production of cordieriteceramic honeycomb structural bodies, fabricated in accordance with theinvention, may be selected from any suitable source. High-purity clay,talc, silica, alumina, aluminum hydroxides and magnesia (MgO)-yieldingraw materials are conventionally used for such ceramics and aresatisfactory here.

The preferred batch materials in commercial use for the production ofvery low expansion extruded cordieritc ceramic bodies are clay, talc,and alumina, with the clays typically constituting kaolinitic clays of aplatey rather than stacked habit. Platey kaolins can be produced by thepreprocessing of stacked kaolinite clays, or the raw material batchincluding the clay can be processed in a way which breaks down thecrystal stacks into platelets.

Any one of a number of known techniques can accomplish the forming ofthe dry batch into a preform or green body suitable for conversion tocordierite by firing. Depending on the porosity desired in thecordierite product, the batch may be mixed with suitable organics andsimply pressed into the shape of a preform, or it may be formed by a hotpressing method.

For the commercial manufacture of flat or thin-walled cordierite ceramicproducts such as ceramic honeycombs, the preferred forming technique isextrusion. A batch mixture suitable for extrusion can be prepared fromthe dry batch by mixing the batch with a suitable liquid vehicle. Thevehicle may comprise water and carbonaceous extrusion aids necessary togive the batch plastic formability and sufficient green strength afterforming to resist breakage prior to firing. Alternatively, the extrusionaids may be mixed with the ceramic batch materials.

The carbonaceous extrusion aids will normally comprise a liquid or solidhydrocarbon material having a vaporization, oxidation or decompositiontemperature of below about 600° C., including for example, organicbinders such as methylcelloluse, carboxymethlcellulose, polyvinylalcohol, alkali stearates, wheat powder, starch paste, glycerin and wax.Batches of this type, which generally contain 20-35% water, aresufficiently plastic so that they can readily be formed by extrusioninto preforms comprising very thin wall dimensions; i.e., less than 1mm. The plasticized batches can also be formed conveniently by rollingor pressing, the rolled or pressed components then being either useddirectly or assembled into more complex shapes prior to firing.

Furthermore, the batch mixture can include other carbonaceous materialssuitable for use as pore-forming agents, including but not limited to,graphite, cherry pit flower, wood chips, saw dust and starch.

In accordance with the method of the present invention, a desirablecordieriteceramic crack-free product is obtained in a two phase firingprocess wherein the green honeycomb structural body is initially firedto a temperature and for a time sufficient to initiate and sufficientlyachieve release of the carbonaceous material. During this initial firingphase the carbonaceous material which has been incorporated in into thegreen honeycomb body is released in either its volatilized or partiallyreacted form. Carbonaceous material, e.g., binder, release typicallyoccurs, depending on the type of organic binder, between about 100-600°C., while, on the other hand, graphite is typically released betweenabout 500-1000° C. As such, this carbonaceous material release phasetypically requires heating to a first temperature either above the firstrange or above the second range, depending on whether or not the ceramicbody contains an amount of higher temperature carbon such as graphite.The release of volatilized or partially reacted carbonaceous materialleads to subsequent undesirable heat release as a result of burning inthe kiln firing space, therefore the initial firing step involvesremoving at least a portion of the undesirable released organic materialat a point proximate to the kiln position in which it is released. Inother words, the inventive method results in preventing the releasedcarbonaceous material from substantially reacting in the firingatmosphere.

After this initial carbonaceous material removal firing phase, theceramic green body is further conventionally fired for a time and atemperature sufficient to initiate and sufficiently achieve theconversion of the green ceramic honeycomb structural body into a firedhoneycomb body whose predominant crystal phase is cordicrite.Temperatures in the range of 1340°-1450° C. are generally suitable forthis purpose when the ceramic material comprises a cordierite containingceramic.

Next, the firing kiln according to the present invention will beexplained in more detail with reference to the attached drawings.

FIG. 2 is a schematic illustrating the construction of an embodiment ofthe tunnel kiln for effecting the firing process according to thepresent invention. In this embodiment the tunnel kiln 10 comprises avestibule region 12, a carbonaceous material release region 14, having aplurality of removal zones (z1-z11), located downstream of the vestibuleregion 12. Each of the removal zones includes a collection trap region(not shown) for collecting and preventing downstream movement ofcarbonaceous material. The kiln further comprises a sintering region 16(partially shown) located downstream of the carbonaceous materialrelease region 14. An exhaust removal system 18, for removing releasedcarbonaceous material, is provided and operatively communicates with theremoval zones of the release region 14.

The exhaust removal system 18 includes a plurality of offtake openings20, specifically, at least one for each removal zone. It is theseofftake openings 20, preferably located in the rooftop of the kiln forthe associated removal zone, through which the released carbonaceousmaterial, either in its volatilized or partially reacted form, isremoved. Each of the offtake openings 20 operatively communicates with asecondary collector conduit 22. Although the embodiment illustratedherein details the offtakes as located in the rooftop, the importantconsideration regarding the location of the offtakes is that they belocated in a position where the volatiles are most easily or efficientlyremoved, which, it should be noted may not always be the rooftop; e.g.,sidewall or a position underneath the kiln.

Regarding the shape of the offtake opening, one skilled in the art canempirically determine, and thus incorporate into the tunnel kilnconfiguration, offtake openings of a shape which is most appropriate forthe optimal and efficient removal of the released carbonaceous removal.

Each of the secondary collector conduits 22 operatively communicateswith a main collector conduit 24. A damper valve 26 is preferablyprovided in each of the secondary collector conduits upstream of thejunction where the secondary and main collector conducts communicate. Anexhaust fan operatively communicates with a main collector conduit 24and functions to place a draw on the kiln firing atmosphere necessaryfor evactuating the released carbonaceous material. Furthermore, adamper valve 28 is provided in the main collector conduit 24. Each ofthe damper valves 26 can be adjusted so as to achieve the properindividual exhaust draw in each of the removal zones z1-z11, and is thisway the removal of the released carbonaceous removal can be shiftedand/or varied from removal zone to removal zone. The overall control ofthe draw on the multiplicity of secondary conduits 26 and associatedofftake openings 20 and the kiln firing atmosphere is controlled byadjustment of the damper valve 28.

The length of the release region and the length of, and the number of,the removal zones are such that they are capable of encompassing thecarbonaceous material release and removal temperature range of a varietyof different compositions and organic material removal requirementswhich changes from composition to composition: i.e., a design which isflexible so as to allow the tailoring of the exhaust profile for anoverall release region which ranges between about 100 to 600° C.

It has been discovered by an examination, that conventional kilnsrelease volatilized and/or partially reacted carbonaceous material, bothof which influence the ability of the conventional kilns to maintaintemperature control. The primary reason for this influence is that thereleased carbonaceous materials burn in the firing atmosphere, proximateto where the material is released and especially in the highertemperature downstream zones where the released material flows as aresult of “crown drift”. This crown drift is caused by hot gases beingdrawn from the higher pressure upstream zones to the lower(morenegative) pressure downstream zones as a result of this pressuredifference between the zone. As these released, combustible carbonaceousmaterials, in their volatile and/or partially reacted form, move intothe higher temperature zones they react with the available oxygen andburn, releasing heat in the process. It is this heat release, which istypically greater than that required by the zone to maintain it'stemperature set point, which causes the temperature in the zone is torise above the desired temperature set point. This is very undesirablebecause as a result of this loss of firing process control, the ceramichoneycomb structural bodies within the kiln become cracked.

The benefit of the inventive kiln described above, is that thisundesirable combustion or heat release is prevented from occurringbecause the kiln is designed so that some or all of this volatized orpartially reacted organic material is removed from the firing atmosphereproximate to the point in the tunnel kiln ware space and thus before thereleased carbonaceous material has a chance to react. Specifically, thereleased, volatized or partially reacted, carbonaceous material, isdrawn, via the exhaust fan, into the offtake openings 20 and through thesecondary collector conduits 22 and thereafter through the maincollector conduits 22 and thereafter through the main collector conduit24 and ultimately through the exhaust fan whereupon it is evactuated.

FIG. 3 is a schematic illustrating the construction of an anotherembodiment of a tunnel kiln for effecting the firing process accordingto the present invention. The same reference numerals in FIG. 2 aregiven to the same or similar parts in FIG. 3, and explanation thereof isomitted. The embodiment of FIG. 3 differs from that of FIG. 2 in thatthe exhaust removal system comprises a series of 4 openings located ineach of the removal zones z1-z11 for removing released, either volatizedor partially reacted, carbonaceous material from the firing atmosphere.A slight modification of this embodiment comprises, rather than a seriesof roof offtake openings, a removal zone incorporating a continuousofftake opening; i.e., a slit in the rooftop which operativelycommunicates with the secondary conduit, for removing the releasedcarbonaceous material.

Referring to FIG. 4A, a partial sectional view of a carbonaceousmaterial release region 14, along line A—A of FIG. 3 Illustrated thereinis a first embodiment of an collection trap region of the inventivetunnel kiln apparatus. Each of the removal regions Z_(n) includes acollection trap region 33 for collecting carbonaceous material. In thisembodiment the collection trap region is comprised of a series ofhanging baffles 44 that extend from the roof portion of, and down into,the removal zones. These hanging baffles 44 are comprised of a materialcapable of withstanding the kiln temperatures in this removal zone;suitable materials include stainless steel, refractory brick, refractoryfiber blanket and/or insulating board materials. The hanging baffles 44are placed such that they form a separate caebonaceous materialcollection trap region 33 for each of the successive removal zones. Inthis embodiment the offtake openings 20 for each of the removal zonesZ_(n) are located in the rooftop of the kiln such that they are incommunication with the collection trap region 33 for each of the removalzones. Note that for the like parts of the embodiment detailed in FIG. 3and explanation thereof is omitted.

Referring to FIG. 4B, a partial sectional view of a carbonaceousmaterial release region 14, along the line A—A of FIG. 3 Illustratedtherein is a second embodiment of a collection trap region for theinventive tunnel kiln apparatus. The same reference numerals in FIG. 4Aare given to the same or similar parts in FIG. 4B, and explanationthereof is omitted. The embodiment of FIG. 4b differs from that of FIG.4A in that the collection trap region 33 is formed via a series of airknife devices that inject a stream of air or other gas (e.g., nitrogen,carbon monoxide etc.) Similar to “hanging” embodiment of FIG. 4A, theair knife device's air jet forms a separate collection trap for each ofthe successive removal zones. As before, offtake openings 20 for each ofthe removal zones are located in the rooftop of the kiln and are incommunication with the carbonaceous material collection trap region 33for each of the respective removal zones.

Referring to FIG. 4C, a partial sectional view of a carbonaceousmaterial release region 14, along line A—A of FIG. 3. Illustratedtherein is a third embodiment of the collection traps for the inventivetunnel kiln apparatus. The same reference numerals in FIG. 4A are givento the same or similar parts in FIG. 4C, and explanation therof isomitted. The embodiment of FIG. 4C differs from that of FIG. 4A in thatthe collection trap region is formed via a series of domed arches or“pockets” that the collection trap region 33 is formed via a series ofdomed arches or “pockets” that are provided in the roof portion of eachof the removal zones. Similar to previous two embodiments of FIG. 4A and4B, each of the removal-zones has a domed portion/“pocket” collectiontrap that is in communication with the offtake openings 20 located inthe rooftop of the kiln.

Each of the three collection trap region embodiments generally functionin the same manner as described in the following: As the ceramic ware 11travels downstream through each of the successive carbonaceous materialrelease region's removal zones Z_(n), in the direction indicated by theware travel arrows, volatized or partially reacted, carbonaceousmaterial is released. The released carbonaceous material indicated byarrow 77 travels upward and into each of the collection trap regions 33where it is trapped and prevented from downstream movement. Aspreviously described, prevention of downstream carbonaceous materialdrift into higher temperature zones avoids undesirable temperature oroxygen level kiln/firing atmosphere conditions. Once collected andtrapped the released carbonaceous material is subsequently drawn intoand through the offtake opening 20 and thereafter exhausted in manner aspreviously described.

Referring again to FIG. 2 the tunnel kiln according to the presentinvention additionally may include an optional conduitreaction-suppression system. It has been observed that releasedcarbonaceous material may condense inside the secondary 22 and maincollector 24 conduits and, if conditions (oxygen level and temperature)are favorable, this material may ignite and burn uncontrolled withinthis conduit space. This uncontrolled burning in the conduit space ductwork is undesirable, thus the inclusion of the conduitreaction-suppresion system, which is designed to aid in the control ofthis uncontrolled conduit carbonaceous material reaction. Thisreaction-suppresion system comprises a temperature monitoring device anda system for introducing a low oxygen content gas into the maincollector conduit 24 and secondary collector conduit 22 when thetemperatures in the conduits are above a predetermined temperature.Preferably, the low oxygen gas comprises either a nitrogen or CO₂enriched gas. The reaction-suppresion system operatively communicateswith the main collector 24 and secondary collector conduits 22communicate. In the tunnel kiln configuration which includes thisreaction-suppression system, the damper valve 28, is located between thejunction where the reaction-suppression system communicates with themain collector conduit 24 and the exhaust fan. When the damper valve 28is fully closed, it functions both to suspend carbonaceous materialremoval and to isolate the secondary conduits 24 from the exhaust fan tofacilitate suppression of undesirable reactions occurring in theconduit.

The reaction-suppression system specifically functions as follows: thetemperature monitoring device, e.g., a thermal sensor in the main ductupstream of the main damper, is connected to an overtemperatureinstrument capable of comparing the duct temperature to a predeterminedset point temperature. When temperature of the thermal sensor reachesthe predetermined set point, which is indicative of burning in the mainand or secondary collector conduits, a damper valve 30 is opened whichreleases a flow of low oxygen gases into the main collector conduit 24which effectively lowers the oxygen level to a point where combustioncan no longer be sustained. Simultaneously, the main collector conduitdamper 28 valve is closed, thus removing the draw to the kiln firingatmosphere. The net result is the suppression of the burning reactionwithin the duct while at the same time stopping the inflow of thereleased and the potentially combustible carbonaceous materials.

It should be noted that the amount of carbonaceous removal from each ofthe individual removal zones and the overall system removal, which isnecessary for achieving the desired firing conditions, will varydepending upon a number of factors including the composition, size andshape of the ceramic body, the ware load, and the size of the cell walland number of cells exhibited by the ceramic body, the kilnconfiguration and the firing schedule utilized. As such, the removalconditions required for achieving the proper firing atmosphere andconditions should be empirically determined for each ceramic/kilnsystem.

As is clear from the above description, according to the ceramichoneycomb structural body firing process of the present invention andtunnel kiln used therefor, the release and removal of the carbonaceousmaterial through the inventive exhaust removal regions and systemsresults in firing conditions in which structural honeycomb bodiesexhibiting a lowered temperature differential between the inner portionand the outer portion of the ceramic body are attained. In other words,firing conditions which are far more conducive for producing firedceramic honeycomb structural bodies which are free of thermaldeformations and thermally induced cracks.

We claim:
 1. A method for firing a green ceramic honeycomb, containingcarbonaceous material, in a tunnel kiln, the method comprising: (a)heating to a first temperature and for a time sufficient tosubstantially release the carbonaceous material from the green ceramichoneycomb; (b) collecting and trapping the released carbonaceousmaterial; (c) removing the released carbonaceous material from theatmosphere; and, (d) heating to a second temperature and for a timesufficient to produce a fired ceramic honeycomb; wherein the removal ofthe released carbonaceous material results in reduced thermally induceddeformation and cracking in the fired ceramic honeycomb.
 2. The methodof claim 1 wherein the fired ceramic honeycomb has a predominant crystalphase of cordierite.
 3. The method of claim 1 wherein the carbonaceousmaterial comprises a liquid or solid hydrocarbon material having avaporization, decomposition or evaporation temperature of below about600° C.
 4. The method of claim 3 wherein the carbonaceous materialcomprises a polymer binder, a hydrocarbon oil or wax binder or graphite.